Nonvolatile semiconductor memory device and method for using the same

ABSTRACT

A nonvolatile semiconductor memory device includes nonvolatile memory cells (C), constant voltage circuits for applying one of different verify voltages to control gates of the nonvolatile memory cells C in response to control data introduced into the memory device from the exterior, and writing and sensing circuit circuits for applying a potential to drains of the nonvolatile memory cells C in response to write data introduced into the memory device and for detecting and amplifying currents between drains and sources of the nonvolatile memory cells. By dividing the memory cell array  501  and a serial register  502  into some parts and by connecting an external SRAM  503  so as to progress the transfer of data from the memory cell array  501  to the serial register  502  and the transfer of data from the serial register  502  to the external SRAM  503  in parallel, the read speed is increased.

This is a divisional of Ser. No. 09/300,402, filed Apr. 27, 1999, whichis a divisional of Ser. No. 09/136,305, filed Aug. 19, 1998, which is adivisional of Ser. No. 08/713,856, filed Sep. 12, 1996, (now U.S. Pat.No. 5,831,900).

BACKGROUND OF THE INVENTION

This invention relates to a nonvolatile semiconductor storage device anda method for using it, and more particularly to a multi-level memory,and its usage, which can store multi-level (multi-value) data in memorycells made of stacked MOS transistors.

A flash memory for storing multi-level data is disclosed in detail inISSCC '95 Digest of Technical Papers, p. 133, for example. The memoryhas an architecture using flash cells as its reference cells to controlthe current flow into the reference cells and to cope with the read-outpotential in accordance with the distribution of threshold values of thecells.

A process for reading data from a NAND flash memory includes randomaccess for reading data of one row of memory cell arrays and for storingit in registers, and subsequent reading of the storage of the registers.When a four-value memory is to be read three times, reading andconversion into a two-value data need the time

3t_(R)+3t_(S)+t_(conv)

where t_(R) is the random access time, t_(S) is the time for readingregisters, and t_(conv) is the time for conversion into a two-valuedata. In a particular case where the random access time t_(R) is 10 μS,the register reading time t_(S) is 25.6 μS for reading 512 bytes in 50nS, and the time t_(conv) for conversion into a two-value data is 5 μS,the total time for reading thrice and for conversion into two valuesamounts in

10×3+25.6×3+5=111.8 μS

The above multi-level memory involves the following problems

(1) A 2″-value memory needs n sense amplifiers. Specifically, afour-value memory needs two sense amplifiers, and an eight-value memoryneeds three sense amplifiers. Thus the multi-level memory requires aLarger area for sense amplifiers.

(2) The number of reference cells is fixed at the time of its design andcannot be flexibly changed later. In a particular case where the numberof reference cells is four, the memory must be a four-value memory evenif the cells are uniform enough to realize a more-value memory. Inanother case where a four-value memory cannot be realized due tovarieties in process parameters during fabrication, although it will beused as a two-value memory, all the sense amplifiers and other circuitelements intended for use with the four-value memory become invalid, andresult in an substantial increase in cost as compared with an originallytwo-value memory.

(3) The area occupied by sense amplifiers is too large to exactly copewith the distribution of cells within the chip.

(4) Reading from a NAND flash memory takes time against the demand forhigh-sped reading.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a nonvolatilesemiconductor storage device for multi-level storage, and a method ofusing it, which is flexibly responsive to the number of multiple levelsand enables minimization of the chip size.

A further object of the invention is to provide a nonvolatilesemiconductor storage device, and a method for using it, which can beread at a high speed.

According to the first aspect of the present invention there is provideda nonvolatile semiconductor memory device comprising:

nonvolatile memory cells each including a source and a drain both formedon one surface of a semiconductor substrate, and including a floatinggate and a control gate which are stacked on the semiconductor substrateabove a portion between the source and the drain via insulation films;

a word line driving circuit for applying one of a plurality ofpredetermined potentials to said control gates of the nonvolatile memorycells, depending on a control data introduced into said memory device;and

writing and sensing circuit for applying a potential to said drains inaccordance with write data introduced into said memory device and fordetecting and amplifying the current flowing between the drain and thesource in each said nonvolatile memory cell.

According to the second aspect of the present invention, there isprovided a method for using a nonvolatile semiconductor memory deviceincluding nonvolatile memory cells which each have a source and a drainboth formed on one surface of a semiconductor substrate, and including afloating gate and a control gate which are stacked on the semiconductorsubstrate above a portion between the source and the drain viainsulation films, comprising:

when a first level of multi-level data including at least a first leveland a second level is to be written, repeating a series of behaviorsuntil writing of said first level is completed, said series of behaviorsincluding: applying a predetermined write voltage between said controlgate and said drain to introduce a charge into said floating gate and tochange the threshold level of said nonvolatile memory cell; applying avoltage responsive to said first level to said control gate anddetecting and amplifying a current flowing between the source and thedrain of said nonvolatile memory cell to perform verify reading; andconfirming whether the writing of said first level is finished or not;

when a second level of said multi-level data is to be written, repeatinga series of behaviors until writing of said second level is completedsaid series of behaviors including: applying said write voltage betweensaid control gate and said drain to introduce a charge into saidfloating gate and to change the threshold level of said nonvolatilememory cell; applying a voltage responsive to said second level to saidcontrol gate and detecting and amplifying a current flowing between thesource and the drain of said nonvolatile memory cell to perform verifyreading; and confirming whether the writing of said first level isfinished or not.

According to the third aspect of the present invention, there isprovided a method for using a nonvolatile semiconductor memory deviceincluding nonvolatile memory cells which each have a source and a drainboth formed on one surface of a semiconductor substrate, and including afloating gate and a control gate which are stacked on the semiconductorsubstrate above a portion between the source and the drain viainsulation films, comprising:

when a first level of multi-level data is to be written, repeating aseries of behaviors until writing of said first level is completed, saidseries of behaviors including: applying a predetermined write voltagebetween said control gate and said drain to introduce a charge into saidfloating gate and to change the threshold level of said nonvolatilememory cell; applying a voltage responsive to said first level to saidcontrol gate and detecting and amplifying a current flowing between thesource and the drain of said nonvolatile memory cell to perform verifyreading; and confirming whether the writing of said first level isfinished or not;

when a second level of said multi-level data is to be written, repeatinga series of behaviors including a first step and a second step untilwriting of said second value is completed, said first step including:applying said write voltage between said control gate and said drain tointroduce a charge into said floating gate and to change the thresholdvalue of said nonvolatile memory cell by a large amount than that forwriting said first value; applying a voltage responsive to said secondvalue to said control gate and detecting and amplifying a currentflowing between the source and the drain of said nonvolatile memory cellto perform verify reading; and confirming whether the writing of saidfirst value is finished or not, and said second step including: applyingsaid write voltage between said control gate and said drain to introducea charge into said floating gate and to change the threshold value ofsaid nonvolatile memory cell; applying a voltage responsive to saidsecond value to said control gate and detecting and amplifying a currentflowing between the source and the drain of said nonvolatile memory cellto perform verify reading; and confirming whether the writing of saidfirst value is finished or not.

According to the fourth aspect of the present invention there isprovided a method for using a nonvolatile semiconductor memory deviceincluding nonvolatile memory cells which each have a source and a drainboth formed on one surface of a semiconductor substrate, and including afloating gate and a control gate which are stacked on the semiconductorsubstrate above a portion between the source and the drain viainsulation films, comprising;

when a first value of multi-level data is to be written, repeating aseries of behaviors until writing of said first value is completed, saidseries of behaviors including: applying a predetermined write voltagebetween said control gate and said drain to introduce a charge into saidfloating gate and to change the threshold value of said nonvolatilememory cell; applying a voltage responsive to said first value to saidcontrol gate and detecting and amplifying a current flowing between thesource and the drain of said nonvolatile memory cell to perform verifyreading; and confirming whether the writing of said first value isfinished or not;

when a second value of said multi-level data is to be written, repeatinga series of behaviors until writing of said second value is completed,said series of behaviors including: while writing said first value insaid nonvolatile memory cell, applying said write voltage between saidcontrol gate and said drain to introduce a charge into said floatinggate and to change the threshold value of said nonvolatile memory cell;applying a voltage responsive to said second value to said control gateand detecting and amplifying a current flowing between the source andthe drain of said nonvolatile memory cell to perform verify reading: andconfirming whether the writing of said first value is finished or not.

According to the fifth aspect of the present invention, there isprovided a nonvolatile semiconductor memory device, comprising:

a memory cell array including a plurality of nonvolatile memory cellsarranged in a matrix in which memory cells in one row are connected by acommon word line and memory cells in one column are connected by acommon bit line;

a first register for holding a first data introduced into said memorydevice;

a word line voltage generating circuit for generating a plurality ofdifferent voltages in response to contents held in said first register;

a second register for holding a second data introduced into said memorydevice;

a word line selecting circuit for selecting said word line in responseto contents held in said second register;

a word line driving circuit for driving said word line selected by saidword line selecting circuit with a voltage generated by said word linevoltage generating circuit;

a plurality of sense amplifier circuits for detecting and amplifying thepotential of said bit line and for holding data corresponding to thepotential of the bit line; and

a column selecting circuit for selectively outputting the data held insaid sense amplifier circuits in response to a third data introducedinto said memory device.

According to the sixth aspect of the present invention, there isprovided a nonvolatile semiconductor memory device, comprising:

a memory cell array including a plurality of nonvolatile memory cellsarranged in a matrix in which memory cells in one row are connected by acommon word line and memory cells in one column are connected by acommon bit line;

a first register for holding a first data introduced into said memorydevice;

a word line voltage generating circuit for generating a plurality ofdifferent voltages in response to contents held in said first register;

a second register for holding a second data introduced into said memorydevice;

a word line selecting circuit for selecting said word line in responseto contents held in said second register;

a word line driving circuit for driving said word line selected by saidword line selecting circuit with a voltage generated by said word linevoltage generating circuit;

a plurality of sense amplifier circuits for detecting and amplifying thepotential of said bit line and for holding data corresponding to thepotential of the bit line;

a column selecting circuit for selectively outputting the data held insaid sense amplifier circuits in response to a third data introducedinto said memory device; and

a plurality of flag cells each associated with a memory cell groupincluding a plurality of memory cells in said memory cell array to holdthe number of data stored in a single memory cell in the associatedmemory cell group.

According to the seventh aspect of the present invention, there isprovided a nonvolatile semiconductor memory comprising:

a memory cell array including a plurality of nonvolatile memory cellsarranged in a matrix in which memory cells in one row are connected by acommon word line and memory cells in one column are connected by acommon bit line;

a plurality of flag cells each associated with a memory cell groupincluding a plurality of memory cells in said memory cell array to holdthe number of data stored in a single memory cell in the associatedmemory cell group;

a word line voltage generating circuit for generating a plurality ofdifferent voltages responsive to contents of said flag cells;

a register for holding an address signal introduced into said memorydevice;

a word line selecting circuit for selecting said word line in responseto contents held in said second register;:

a word line driving circuit for driving said word line selected by saidword line selecting circuit with a voltage generated by said word linevoltage generating circuit;

a plurality of sense amplifier circuits for detecting and amplifying thepotential of said bit line and for holding data corresponding to thepotential of the bit line; and

a column selecting circuit for selectively outputting the data held insaid sense amplifier circuits in response to a third data introducedinto said memory device.

According to the eighth aspect of the present invention, there isprovided a method for using a nonvolatile semiconductor memory devicewhich includes a memory cell array having a plurality of nonvolatilememory cells arranged in a matrix in which memory cells in one row areconnected by a common word line and memory cells in one column areconnected by a common bit line, and a plurality of flag cells eachassociated with a memory cell group including a plurality of memorycells in said memory cell array to hold the number of data stored in asingle memory cell in the associated memory cell group, comprising thesteps of:

reading a flag data of a flag cell of a memory cell group to which amemory cell to be read belongs;

repeating a plurality of cycles of driving a word line connected to thememory cell with a predetermined voltage pursuant to the flag data ofsaid flag cell, then detecting and amplifying a voltage of a bit line,and outputting the read-out data; and

converting the data read out in said cycles into binary data.

According to the ninth aspect of the present invention, there isprovided a nonvolatile semiconductor memory device comprising:

a memory cell array including a plurality of nonvolatile memory cellsarranged in a matrix in which memory cells in one row are connected by acommon word line, and memory cells in one column are connected by acommon bit line, and data of one row can be read out for each divisionalpart thereof;

a serial resister aligned in parallel with said word lines in saidmemory cell array and capable of storing and reading individualdivisional parts of data of the memory cell array independently; and

a memory for storing data from said serial register for each divisionalpart.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a multi-level memory according to anembodiment of the invention;

FIG. 2 is a diagram showing changes in threshold value during a writingprocess in the memory according to the embodiment of the invention;

FIG. 3 is a diagram showing changes in threshold value during anotherwriting process in the memory according to the embodiment of theinvention;

FIG. 4 is a diagram showing changes in threshold value during anotherwriting process in the memory according to the embodiment of theinvention;

FIG. 5 is a circuit arrangement of a nonvolatile semiconductor storagedevice according to the embodiment of the invention;

FIG. 6 is a circuit diagram of memory cells according to the embodimentof the invention;

FIG. 7 is a circuit diagram of a major part of the embodiment of theinvention;

FIG. 8 is a circuit diagram of a major part of the embodiment of theinvention;

FIG. 9 is a circuit diagram of a major part of the embodiment of theinvention;

FIG. 10 is a circuit diagram of a major part of the embodiment of theinvention;

FIG. 11 is a table of voltages applied during operation of the circuitshown in FIG. 10;

FIG. 12 is a flow chart of a reading operation in the embodiment of theinvention;

FIGS. 13A and 13B are diagrams showing a modification of the invention;

FIG. 14 is a diagram of another modification of the invention;

FIG. 15 is a diagram of still another modification of the invention;

FIG. 16 is a diagram of a modification of the invention during a readingprocess;

FIG. 17 is a circuit diagram of yet another modification of theinvention;

FIG. 18 is a diagram of a modification of the invention using thecircuit of FIG. 17 during a reading process;

FIG. 19 is a table showing operations of an encoder/decoder circuit;

FIGS. 20A and 20B are a circuit arrangement of the encoder/decodercircuit;

FIG. 21 is a block diagram of an embodiment improved in reading speed;

FIG. 22 is a perspective view of a memory board realizing thearrangement of FIG. 21;

FIGS. 23A and 23B are a perspective view and a front elevational view ofanother memory board realizing the arrangement of FIG. 21;

FIG. 24 is a perspective view of still another memory board realizingthe arrangement of FIG. 21; and

FIGS. 25A-25G are explanatory diagrams of a reading operation in theembodiment shown in FIG. 21.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A general aspect of a nonvolatile semiconductor storage device accordingto the invention is explained below with reference to FIGS. 1 through 4.

In FIG. 1, SA0 through SAn refer to sense amplifiers having a bit-to-bitverify function.

Memory cells C0 to Cn are nonvolatile memory cells made of two-layeredgate MOS transistors which each have a source, a drain, both formed on asurface of a semiconductor substrate, a floating gate and a controlgate, which are stacked on the source and the drain via an insulationfilm. In this embodiment using NAND memory cells as memory cells C0 toCn, control gates of memory cells in a line are connected to a commonword line WL.

The nonvolatile memory cells store multi-level information with two ormore values depending upon values of threshold voltages variablycontrolled by injecting charges into their floating gates. For example,in a particular case where the word line driving voltage is 5V, bydetermining a threshold value approx. −1V as “11” (“3”), another approx.1.5V as “10” (“2”), another approx. 3V as “01” (“1”), and anotherapprox. 4.5V as “00” (“0”), then the memory cells behave as four-valuestorage means. If a threshold value around −1V is regarded as “111”(“7”), another approx. 0.6V as “110” (“6”), another approx. 1.2V as“101” (“5”), another approx. 1.8V as “101” (“4”), another approx. 2.4Vas “011” (“3”), another approx. 3.0V as “010” (“2”), another approx.3.6V as “001” (“1”), and another approx. 4.2V as “000” (“0”), then thememory cells behave as eight-value storage means. The number of multiplelevels may be other than 2″, such as five-value memory cells, in which athreshold value approx. −1V may be defined as “100” (“4”), anotherapprox. 1V as “011” (“3”), approx. around 2V as “010” (“2”), anotherapprox. 3V as “001” (“1”), and another approx. 4V as “000” (“0”).Nevertheless, n multiples of 2 are recommended for easier conversionafter reading.

Numerals 200 through 202 denote constant voltage generating circuitswhich output constant verify voltages Vvfy1, Vvfy2, Vvfy3, respectively.For storing four-value data in the memory cells as mentioned above, Vvfy1 may be set to 1.5V, Vvfy2 to 3.0V, and Vvfy3 to 4.5V. Actually, muchmore constant voltage circuits or reference voltage generating circuitsare provided as explained later, so that multi-level storage from twovalues to eight values, for example, can be variably, selectivelyeffected by executing writing and reading while setting the word line toa chosen potential.

Numeral 203 denotes a write voltage generating circuit for supplying theword line with a voltage Vpp (for example, the constant voltage of 20V,or a voltage which stepwise increases by a predetermined value, like16V, 16.5V and 17V, every occurrence of writing.

Next explained is a writing process in the circuit of FIG. 1 withreference to FIG. 2. For four-value storage, writing operations in threesteps. Before writing, erase is already finished, and the thresholdvalue of memory cells is set to −1V, for example. The value correspondsto “11” (“3”). FIG. 2 shows the progress of the program time and changesin threshold value with time of both a fast cell, in which writing isfast, and a slow cell, in which writing is slow because its insulationfilm is thick.

(Step 1): The following sub-steps 1-1 through 1-5 are repeated.

1-1: First data of a decoded value of write data is set in a write datalatch in the sense amplifier. If the decoded value is “11” (“3”), thenthe first value is “1”. If “10” (“2”), then “0”. If “01” (“1”), then“1”. If “00” (“0”), then “1”. “1” corresponds to the write prohibitvoltage, and “0” corresponds to the write voltage. That is, the writevoltage is applied only to the bit line corresponding to the memory cellin which “10” is to be written, and the write prohibit voltage isapplied to the bit lines corresponding to the other memory cells.

1-2: Vpp (20V) is applied to the word line WL, and writing is executedfor the write time tp. More specifically. the voltage, 20V, is appliedto the word line, the write voltage, 0V, is applied to the bit linecorresponding to the memory call for writing “10”, and the writeprohibit voltage, 10V, is applied to the bit lines corresponding to theother memory cells. As a result, only the threshold value of the memorycell for writing “10” slightly increases, and the threshold values ofthe other memory cells remain unchanged.

1-3: The word line WL is driven up to Vvfy1 (1.5V) to execute a verifyoperation. That is, all bit lines are precharged to 5V, and a timelater, sense amplifiers detect the potentials of the bit lines. If thewriting is finished, the write data latches in the sense amplifiers areinverted from “0” to “1”.

1-4: Detection is done whether data is “1” in all of the write datalatches. If so, the process ends.

1-5: If data is not “1” in one or more write data latches, Vpp isincreased by ΔVpp (for example, 0.5V) or the write time tp is increasedby Δtp to prepare for subsequent steps 1-1 et seq. This step, however,may be omitted.

(Step 2): The following sub-steps 2-1 through 2-5 are repeated.

2-1: Second data of the decoded value of the write data is set in awrite data latch in the sense amplifier. If the decoded value is “11”,then the second value is “1”. If “10”, then “1”. If “01”, then “0”. If“00”, then “1”. That is, the write voltage is applied only to the bitline corresponding to the memory cell in which “01” is to be written,and the write prohibit voltage is applied to the bit lines correspondingto the other memory cells.

2-2: Vpp (20V) is applied to the word line WL, and writing is executedfor the write time tp. As a result, only the threshold value of thememory cell for writing “01” slightly increases, and the thresholdvalues of the other memory cells remain unchanged.

2-3: The word line WL is driven up to Vvfy2 (3.0V) to execute a verifyoperation. That is, all bit lines are precharged to 5V, and a timelater, sense amplifiers detect the potentials of the bit lines. If thewriting is finished, the write data latches in the sense amplifiers areinverted from “0” to “1”.

2-4: Detection is done whether data is “1” in all of the write datalatches. If so, the process ends.

2-5: If data is not “1” in one or more write data latches, Vpp isincreased by ΔVpp (for example, 0.5V) or the write time tp is increasedby Δtp to prepare for subsequent steps. This step, however, may beomitted.

(Step 3): The following sub-steps 3-1 through 3-5 are repeated.

3-1: Third data of the decoded value of the write data is set in a writedata latch in the sense amplifier. If the decoded value is “11”, thenthe third value is “1”. If “10”, then “1”. If “01”, then “1”. If “00”,then “0”. That is, the write voltage is applied only to the bit linecorresponding to the memory cell in which “00” is to be written, and thewrite prohibit voltage is applied to the bit lines corresponding to theother memory cells.

3-2: Vpp (20V) is applied to the word line WL, and writing is executedfor the write time tp. As a result, only the threshold value of thememory cell for writing “00” slightly increases, and the thresholdvalues of the other memory cells remain unchanged.

3-3: The word line WL is driven up to Vvfy3 (4.5V) to execute a verifyoperation. That is, all bit lines are precharged to 5V, and a timelater, sense amplifiers detect the potentials of the bit lines. If thewriting is finished, the write data latches in the sense amplifiers areinverted from “0” to “1”.

3-4: Detection is done whether data is “1” in all of the write datalatches. If so, the process ends.

3-5: If data is not “1” in one or more write data latches, Vpp isincreased by ΔVpp (for example, 0.5V) or the write time tp is increasedby Δtp to prepare for subsequent steps. This step, however, may beomitted.

It is understood that multi-level writing is done by execution of theabove three steps. In short, data of a decoded value are, in the orderof the first, second and third data, “111” for “3”, “011” for “2”, “101”for “1”, and “110” for 0″. In case of three-value storage, “2” isdecoded into “11”, “1” into “01”, and “0” into “10”. In case offive-value storage, “4” is decoded into “1111”, “3” into “0111”, “2”into “1011”, “1” into “1101”, and “0” into “1110”. Generalizing it byusing n, “n” is decoded into “1111 . . . 11”, “n-1” into “0111 . . .11”, “n-2” into “1011 . . . 11”, “1” into “1111 . . . 01”, and “0” into“1111 . . . 10”. The threshold value is lowest for “n” and highest for“0”.

In this process decoding writing data as explained above and using thedecoded data sequentially as write data so as to increase the thresholdvalues of memory cells little by little and to give the word line apotential corresponding to the write data for execution of write verify,it is possible to store any desired multi-level in a single memory cell.For reading, a slightly lower potential than the verify potential issupplied to the word line. Thus, “3” is read out as 000″, “2” as “100”,“1” as “110”, and “0” as “111”, in sequence, and they are encoded toform a multi-level data. In general, “n” read as “0000 . . . 00”, “n-1”read as “1000 . . . 00”, “n-2” read as “1100 . . . 00”, “1” read “1111 .. . 10”, and “0” read as “1111 . . . 11” are encoded into a multi-data.

By employing the above-explained write and read system, the followingeffects are obtained.

1. The area for sense amplifiers may be small. That is, a single senseamplifier is sufficient regardless of the numbers of multiple levels.

2. Since a plurality of constant voltage circuits 200 through 202 areused in lieu of reference cells, the number of multiple levels isvariable.

As a result, an architecture for multi-level flash memory can beprovided which can flexibly cope with any number of multiple levels andcan minimize the chip size.

Shown in FIG. 3 is another write method. This method employs the samewrite data decode method as shown above, but enhances the writing infirst cycles in steps 2 and 3 (by increasing the initial value of Vpp orby elongating the initial value of the write time). As a result, thetotal write time can be reduced.

Shown in FIG. 4 is still another write method. This method employs adifferent way of decoding. That is, for writing data for four-valuestorage, “3” is decoded into “111”, “2” into “011”, “1” into “001”, and“0” into “000”. In general, “n” is decoded into “1111 . . . 11”, “n-1”into “0111 . . . 11”, “n-2” into “0011 . . . 11”, “1” into “0000 . . .01”, and “0” into “0000 . . . 00”. As a result, the write time isreduced as shown in FIG. 4.

Next explained are details of a circuit arrangement of an embodiment ofthe invention with reference to FIG. 5. The memory system according tothe embodiment generally comprises a multi-level memory 100, CPU 300,RAM 301, and flag data memory 302. These elements are connected by a bus120, control signal line 121, and so forth.

The multi-level memory 100 includes a word line drive voltage data latchcircuit 101, row address latch circuit 102, column address latch circuit103, word line drive voltage generating circuit 104, row decode circuit105, word line drive circuit 106, column decode circuit 107, memory cellarray 108, precharge circuit 109, sense amplifier array 110, column gate111, and so forth.

The bus 120 has a bit width of, for example, eight bits. The word linedrive voltage data latch circuit 101, row address latch circuit 102, andcolumn address latch circuit 102 are of eight-bit latch.

The memory cell array 108 has the capacitance of 64M elements made byarranging two-layered gate MOS transistors in a matrix with 32K rows and2K columns. The memory cells have a NAND arrangement.

FIG. 6 shows an arrangement of NAND memory cells. The NAND memory cellsare made up of a MOS transistor Q41 whose gate is driven by a drain-sideselect gate line SG1, two-layered gate MOS transistors Q42 to Q45 whosegates are driven by word lines WL, and MOS transistor Q46 whose gate isdriven by a source-side select gate line SG2. A single memory cell array108 includes 2K rows and 2K columns of NAND bundles. Memory cells in onerow (memory cells connected to a single word line) forms one page (2Kbits), and NAND bundles aligned in the direction of columns form oneblock (32K bits). A single 64M-bit memory cell array includes 2K blocks.

An arrangement of the memory cell array 108, precharge circuit 109,sense amplifier array 110, column gate circuit 111, and so on, is shownin FIG. 7. As explained above, the arrangement includes 2K bit lines;however, the drawing shows only three bit lines. The precharge circuit109 is composed of a P-type MOS transistor Q3 controlled by a signal φ1.NAND bundles 2-1 to 2-3 have the arrangement shown in FIG. 6. The senseamplifier array 110 includes a plurality of sense amplifier circuitsS/A. Each sense amplifier circuit is made up of flip-flop circuits 1-1to 1-3, N-type MOS transistors Q2, Q4, Q7, Q8, and includes batch verifydetecting circuit made of Q201 through Q203, Q21 and inverter circuit20. The column gate circuit 111 selectively connects the flop-flopcircuits to a IO line Pair connected to the bus 120 under control ofcolumn selecting lines CS.

The flip-flop circuits 1-1 to 1-3 behave as write data latches and readdata latches. That is, they behave as write data latch during datawriting.

Writing is effected in the following steps. For an increase of thethreshold value of memory cells (program), i.e. for writing “0”, “L” issupplied to IO, and “H” to BIO (IO line having negative logic). Thus,the node N1 of the flip-flop circuit 1-1 is set to “L”, and BN1 to “H”.After that, the source level of the flip-flop is increased to give theincreased “H” level to the signal φ2. Thus, the MOS transistor Q4 ismade conductive, and the bit line Becomes 20V. After that, by increasingthe voltage of the word line to 20V, the potential difference betweenthe control gate of the selected memory cell and the channel becomes20v, and the threshold value increases due to entry of electrons intothe floating gate.

After that, the following verification operation is effected. The signalφ1 is set to “L” level to make the MOS transistor Q3 conductive and toprecharge the bit line BL with 5V. Then, a potential of a verify level(determined between 0V to 5V depending on the write multi-level data asexplained above) is supplied to the word line WL of the written memorycell. A time later, the charge on the bit line is discharged the groundterminal (common source line CSL) depending on the threshold value ofthe memory cell. The MOS transistor Q8 is conducted by setting thesignal φ3 to “H” to detect the potential of the bit line BL at thistime. As a result, conduction of the MOS transistor Q7 is controlleddepending on the bit line potential so that the flip-flop is inverted ifthe bit line maintains the “H” level, but is not inverted if the bitline potential is already discharged. This is responsive to theflip-flop being inverted upon completion of writing. As alreadyexplained, the foregoing writing behaviors are repeated bit by bit tosequentially increase the threshold value of the memory cell.

When the threshold value of a memory cell is held constant, that is, forwriting “1”, “H” is supplied to IO, and “1” to BIO. Thus, the node N1 ofthe flip-flop circuit 1-1 is set to “H”, and BN1 to “1”. After that, thesource level of the flip-flop in increased, and the increased “H” level(10V) is given to the signal φ2. Then, the MOS transistor Q4 is madeconductive, and the bit line is set to 10V. After that, by increasingthe voltage of the word line to 20V, the potential difference betweenthe control gate of the selected memory cell and the channel becomes10V, and the threshold value maintains a constant value becauseelectrons are block out from the floating gate. In the following verifyoperation, the flip-flop maintains the latest status regardless of anypotential of the bit line. These behaviors are conducted not only forwriting “1” but also in steps taken after writing “0” is completed.

Detection whether the writing is finished is done by the batch verifydetecting circuit. That is, by giving the “L” pulse to φ5 and by makingthe MOS transistor Q21 conductive, the common verify line 26 isprecharged to “H”. If writing is not finished in one or more cells, thenode BN exhibits “H” level because the node BN of a cell in whichwriting is not done or completed should exhibit the “L” level.Therefore, if there is any cell in which writing is not completed, oneof the MOS transistors Q201 through Q203 is conducted , and the commonverify line 26 exhibits “L”. When writing is completed in all of thecells, then the common verify line 26 exhibits “H”. In this manner, aslong as the output VFY of the inverter circuit 20 maintains “H”, writingis not yet completed, and VFY is changed to “L” only when writing iscompleted.

Reading is done in the following steps.

A “H” pulse is given as signal φ6, N1 is set to “L”, and BN1 to “H”.After that, signal φ1 is set to the “L” level to makes the MOStransistor Q3 conductive, and the bit line BL is precharged to 5V. Then,the read level potential (determined between 0V to 5V depending on thewrite multilevel data as explained above) is supplied to the word lineWL of the selected memory cell. A time later, the charge on the bit lineis discharged to the ground terminal (common source line CSL) dependingon the threshold value of the memory cell. The MOS transistor Q8 isconducted by setting the signal φ3 to “H” to detect the potential of thebit line BL at this time. As a result, conduction of the MOS transistorQ7 is controlled depending on the bit line potential so that theflip-flop is inverted if the bit line maintains the “H” level, but isnot inverted if the bit line potential is already discharged. In thismanner, when the threshold value of the memory cell is higher than thepotential of the word line, the bit line level becomes “L”, the MOStransistor Q7 is not conducted, and the node N1 maintains “L”. This iscalled “0” reading.

When the threshold value of the memory cell is higher than the potentialof the word line, the bit line level becomes “H”, the MOS transistor Q7is conducted, and the node N1 exhibits the “H” level. This is called “1”reading.

The word line drive voltage generating circuit 104 has the arrangementshown in FIG. 8, which includes a decode circuit 151, reference voltagegenerating circuit 150, transfer gate circuit 152, current mirrorcomparator circuit 153, drive inverter circuit 154, and so forth.

The decode circuit 151 decodes data d1 to d8 latched in the word linedrive voltage data latch circuit 101 and generates 256 outputs (whichmay be less).

The reference voltage generating circuit 150 is made by seriallyconnecting plural steps of resistor elements R, and outputs apredetermined potential by resistive division.

The divided reference potential output is supplied through 256 transfergates (which may be less) to a source follower circuit formed of thecurrent mirror comparator circuit 153 and the drive inverter circuit154. In this process, a word line drive voltage VWL is generated.

Next explained are details of the row decode circuit 105. The row decodeCircuit 105 employs a partial decode method, and includes an intra-blockdecode circuit RD1 and a block decode circuit RD2. FIG. 9 shows detailsof a circuit arrangement of the intra-block decode circuit RD1. Thiscircuit includes a decode portion for decoding a row address R.Add and aCG drive circuit 702 supplied with by the word line drive voltage VWL.The intra-block decode circuit RD1 determines whichever word line is tobe selected among the NAND bundles.

FIG. 10 shows details of a block decode circuit RD2 and the word linedrive circuit 106.

The block decode circuit RD2 decodes a row address is R.Add (which isdifferent from the row address input to the intra-block decode circuitRD1 and may be, for example, an upper address), and selects a block.

The word line drive circuit 106 is made of transfer gates 401, 402, 410,MOS transistors Q134, Q135, Q136, Q121, Q122. Q131, Q132, Q133, levelshifter 709, word line ground circuit 411, and so forth.

The word line drive circuit 106 of the block selected by the blockdecode circuit RD2 drives word lines WL1 through WL16 in response to CG1to CG16 signals. Potentials applied to signals A to E and power sourcesVA, VB, VC are shown in FIG. 11. Vpp represents 20V, Vm 10V, Vcc 5V, andGND 0V.

Returning back to FIG. 5, RAM 301 is for temporarily storing data, etc.to be written, and the flag data memory 302 is a nonvolatile memory forstoring the number of multiple levels of respective blocks in the memorycell array 108. In case of a memory cell array having a smallcapacitance, characteristics of a single memory cell array is considereduniform, and the maximum number of multiple levels is consideredconstant in any cell. In contrast, in 64M memory cells, for example,there may possibly be a variance in maximum number of multiple levelsdepending upon the locations of the memory cells, but multiple levelsare considered substantially the same in adjacent memory cells. Sincethe flag data memory 302 stores multiple levels of respective blocks inthe memory cell array 108, multiple levels can be set independently inindividual blocks. For example, blocks 1 to 20 can be used as athree-level memory, blocks 20 to 40 as a four-level memory, blocks 40 to2000 as a five-level memory, blocks 2000 to 2020 as a four-level memory,and blocks 2020 to 2048 as a three-level memory. CPU 300 executesread/write control, data transfer control, write data decoding, readdata encoding (data conversion), and so forth.

When all memory cells in one chip are used for the same multi-levelstorage, no flag data memory is necessary. For example, if they are usedas a four-level memory, CPU may control such that both reading andwriting are always performed in three steps.

Referring to FIG. 12, behaviors of the memory system shown in FIG. 5 areexplained below. FIG. 12 is a flow chart of a reading process.

Flag data is first read out (S001). If, for example, data of memorycells in the block 1 is to be read, flag data in the flag data memory302 corresponding to the block is read out. The flag data indicates themulti-level number n of the block. For example, if the memory cells inthe block 1 is a three-level memory, then n is 3. After that, CPU 300latches data indicating the first verify voltage corresponding to themulti-level number n in the word line drive voltage data latch circuit101 (S002). Then, the word line drive voltage generating circuit 104outputs 1.8, for example, corresponding to the data. Next, a row addressis input (S003). Since it requires more than eight bits, the addressdata must be transferred in two cycles. After that, the word line isdriven by 1.8V to perform a sense operation (S004). As a result, if thestored data is “2”, then “0” is latched in the flip-flop in the senseamplifier. If it is “1”, then “1” is latched. If it is “0”, then “1” islatched. The data is sequentially read out by changing the columnaddress, and stored in RAM 301 (S005). These steps S002 to S005 arerepeated once again (normally n-1 times).

The second cycle is progressed in the same manner except that the wordline voltage is 3.6V. As a result, if the stored data is “2”, “0” islatched in the flip-flop in the sense amplifier. If it is “1”, then “0”is latched. If it is “0”, then “1” is latched. Then, the data is storedin RAM 301.

After that, a stream of bits is generated by encoding the data stored inRAM 301. An algorithm for converting a ternary number (three-bitinformation) into a binary number is used (S006). In this example, CPUand software for controlling the CPU are used to execute encode anddecode operations; however, these operations may be done by hardware asshown in FIGS. 19, 20A and 20B.

FIG. 19 is a table showing relations between a stream of data bits D0,D1 and a stream of data d0, d1, d2 which are decoded from D0, D1, storedin RAM 301 and transferred to the memory cell array. FIGS. 20A and 20Bare circuit diagrams for realizing an encode operation and a decodeoperation. The circuit of FIG. 20A is for realizing a decode operationand includes an AND circuit AND11, OR circuit OR11, and so forth. Thecircuit of FIG. 20B is for realizing an encode operation and includesAND circuits AND 12, AND13, OR circuit OR12, inverter circuits INV11,INV12, and so forth.

FIGS. 13A and 13B show a system arrangement with a large capacitanceusing the above system. FIG. 13A shows an example using a plurality ofchips, in which the multilevel memory 100, CPU 300, RAM 301, flag datamemory 302 among elements in the circuit of FIG. 5 are incorporated intoeach single chip. FIG. 13B shows another example in which the controllerportion (CPU 300, RAM 301 and flag data memory 302) is disposed onanother single chip. When the encoder/decoder circuits of FIGS. 20A and20B are used, they should be shared as a common control circuit.

Other than this arrangement, a number of multi-level memory chips may bemounted on a board or a card while the other elements including thecontroller is incorporated on another board. In this case, by using thecontroller commonly, the storage capacitance can be increased asdesired. Of course, the controller may be composed of plural chips.

FIG. 14 shows an example in which one block is for one word line. Theblock is smaller than that of FIG. 5, and the total amount of data thatcan be stored in one chip can be increased.

FIG. 15 shows an example in which each chip has the flag data memory302. This arrangement facilitates extension of the system. If memorycells in one chip are not divided into blocks and always used forstorage of the same multi-level number, then a single nonvolatileregister is sufficient in one chip to store in the nonvolatile registera multi-level number common to all memory cells in one chip.

A memory system having the arrangement shown in FIGS. 13A, B through 15will be brought into commerce in a unique way different from theconventional sale.

In a first possible case, the manufacturer probably tests themulti-level number of each chip. According to the result of the test,certain multi-level numbers within a guaranteed range are previouslystored in nonvolatile multi-level registers.

In a second possible case, memory cells in one chip are probably dividedinto blocks. If the memory should be designed to use different blocksfor different multi-level numbers, multi-level numbers will bepreviously stored in respective blocks with reference to the result of atest by the manufacturer.

In a third possible case, all of the above test may be relied on user'sjob. In this case, a user determines multi-level numbers on his ownresponsibility.

Through these ways of sale, it will be possible to supply a much lessexpensive memory than other type memories.

As described above, when CPU 300 (decoder/encoder) is providedexternally of RAM chips, the performance is inevitably deterioratedbecause three read cycles must be repeated for four-level storage andseven read cycles for eight-level storage. FIG. 16 shows a read processimproved in this respect.

First, bit lines are precharged to set word lines to a highest Vvfylevel. At time t1 when cells of “11” discharge the bit lines, the φ3pulse of sense amplifiers is made to rise, and corresponding informationof the bit lines is latched. At this time, since sense amplifiers otherthan “11” are inverted from the reset status, bit lines corresponding to“11” can be known. By using this method for all sense amplifiers,information is read out and stored in RAM 302 in the controller beforecells of “10” discharge the bit lines. By executing it for “10” and “01”as shown in FIG. 16, all information can be read by a single precharge.

This method of reading must read out all information within a time fordischarging data of adjacent cells, and involves a difficulty in respectof timing. As a result, along with an increase in amount of information,a significantly high speed is required for serial access. FIG. 17 showsa countermeasure in this respect.

In this arrangement, two sense amplifiers are provided for each bit linefor two-way access. That is, the sense amplifier S/A1 reads datacorresponding to “11” and “01” alone, and “10” between them is latchedby the sense amplifier S/A2. Although this arrangement needs two couplesof data bus lines, a single couple of data bus lines is also acceptableif they are alternatively made to appear on the data bus by appropriateswitching of the column gates.

In this case, if the same method is used for data load during program,two MOS transistors, Q4 and Q4′, must be used. However, in aspecification where slower data load is acceptable, bit-to-bit verifywriting may be done by the sense amplifier S/Al alone, and the MOStransistor Q4′ may be omitted.

FIGS. 21 through 25 show still further embodiments of the invention,which are improved in read speed.

FIG. 21 is a block diagram schematically showing a nonvolatilesemiconductor memory device according to the invention. As shown, thedevice includes a RAM portion 501 in which NAND flash memory cells arearranged in a matrix, register 502 aligned in parallel with word linesof the RAM portion, external SRAM 503 for storing contents of theregister, and code converting portion 504 for converting read-out datainto binary data. The RAM portion 501 is a multi-level (maximum value isn) memory of 1K word by 1K bits, and the register also has the capacityof 1K bits. The register is divided into two parts of 512 bits asexplained later. The external SRAM has the capacity of 1K bits x (n-2)rows.

FIG. 22 is a perspective view of the outer aspect of a memory board asan actual memory device which includes a plurality of RAM chips 602, CPUchip 603 and SRAM chip 604 mounted on a major surface of a substrate601.

FIGS. 23A and 23B show another example of the memory board, in whichFIG. 23A is a perspective view taken from the front surface and FIG. 23Bis a front elevational view. In this example, a plurality of RAM chips602 are mounted on the front surface of a substrate 611, and the CPUchip 603 and the SRAM chip 604 are mounted on the back surface.

FIG. 24 shows still another example in which the CPU chip 603, SRAM chip604 and connector 622 are mounted on a mother board substrate 621, and amemory board having a plurality of RAM chips 602 is connected to theconnector 622.

Although the devices shown in FIGS. 22 through 24 are realized in formof memory boards, they may be realized as standardized memory cards.

FIGS. 25A-25G show operations of the embodiments when four-level data(n=4) to be stored in the RAM.

Here is used a scheme of pipeline. Data VWL0 of a word line WL0 is firstread and stored in the register 502 (FIG. 25A). Then, the VWL0 data ofthe least-significant half of the register 402 (least-significant 512bits) is serially read out and transferred to the first row of theexternal SRAM 503 (FIG. 25B). Next, the row address is progressed, anddata VWL1 of the word line WL1 is read out. In this step, only theleast-significant half is read and stored in the least-significantportion of the register 502. At the same time, VWL0 data stored in themost-significant portion of the register 502 is transferred to the firstrow of the external SRAM 503 (FIG. 25C). After that, theleast-significant half VWL1 data stored in the register 502 istransferred to the second row of the external SRAM 503, and themost-significant portion of VWL1 data is stored in the most-significanthalf of the register 502 (FIG. 25D). By progressing the row address ofthe RAM portion 501 once again, while the most-significant half of VWL1data stored in the register 502 is transferred to the second row of theexternal SRAM 503, the least-significant portion of VWL2 data is storedin the least-significant half of the register 502 (FIG. 25E). Next,while the most-significant half of VWL2 data is stored in themost-significant portion of the register 502, the least-significant partof VWL2 data is sent to the code converter 504 together with theleast-significant, part of data in the external SRAM 503 where everythree bits are converted into a binary number simultaneously (FIG. 25F).Similarly, the second half of VWL2 data stored in the first half of theregister 502, together with the first part of the data in the externalSRAM 503, is converted into binary numbers in the code converter 504(FIG. 25G).

Since both access and read are progressed in parallel in this manner,under the relation of t_(R)<t_(S)/2, the total read time is reduced to

t_(R)+3t_(S)+t_(CONV)

Moreover, in the arrangement shown in FIG. 25 preparing the externalSRAM for some bits, if new data is read and output in synchronizationwith preceding two-level information stored there and they are convertedsimultaneously, the time t_(CONV) is reduced into a fractional level.

Additionally, since the amount of information is doubled after codeconversion, a higher throughput can be obtained by doubling the readclock, i.e. in the case that serial access is performed in 50 ns, byeffecting outputting from the code converter 504 into 25 ns in the aboveexample.

Although the examples of FIGS. 22 through 24 use an independent SRAMchip, a high-speed cache memory attached to CPU may be used.

The memory cells and the register may be divided into three or moreparts instead of two parts employed in the embodiment shown above. Then,the read efficiency can be further improved by optimum control of theread start address.

Thus, the invention can realize a multi-level flash memory that canflexibly determine the multiple levels in accordance of the actual powerof cells and can minimize the chip size. The invention is not limited tothe embodiments shown above, but involves various alterations andmodifications without departing from the spirit and scope of theinvention. The invention is applicable also to NOR memory cells in lieuof NAND cells shown and described above.

As explained above, according to the invention configured to flexiblyuse memory cells depending upon available multiple levels and a mode ofuse, considering that the availability of memory cells for multiplelevels varies within an array or between chips, an inexpensivemultilevel flash memory minimizing the chip size can be realized.

Additionally, reading of data from a divisional part of a memory cellarray to a divisional part of a register is executed in parallel withtransfer of data from another divisional part of the register to anexternal SRAM, the read speed can be increased.

What is claimed is:
 1. A method for using a nonvolatile semiconductormemory device including nonvolatile memory cells which each have asource and a drain both formed on one surface of a semiconductorsubstrate, and including a floating gate and a control gate which arestacked on the semiconductor substrate above a portion between thesource and the drain via insulation films, comprising: when a firstlevel of multi-level data including at least a first level and a secondlevel is to be written, repeating a series of behaviors until writing ofsaid first level is completed, said series of behaviors including:applying a predetermined write voltage between said control gate andsaid drain to introduce a charge into said floating gate and to changethe threshold level of said nonvolatile memory cell; applying a voltageresponsive to said first level to said control gate and detecting andamplifying a current flowing between the source and the drain of saidnonvolatile memory cell to perform verify reading; and confirmingwhether the writing of said first level is finished or not; when asecond level of said multi-level data is to be written, repeating aseries of behaviors until writing of said second level is completed,said series of behaviors including: applying said write voltage betweensaid control gate and said drain to introduce a charge into saidfloating gate and to change the threshold level of said nonvolatilememory cell; applying a voltage responsive to said second level to saidcontrol gate and detecting and amplifying a current flowing between thesource and the drain of said nonvolatile memory cell to perform verifyreading; and confirming whether the writing of said first level isfinished or not.
 2. A method for using a nonvolatile semiconductormemory device including nonvolatile memory cells which each have asource and a drain both formed on one surface of a semiconductorsubstrate, and including a floating gate and a control gate which arestacked on the semiconductor substrate above a portion between thesource and the drain via insulation films, comprising: when a firstlevel of multi-level data is to be written, repeating a series ofbehaviors until writing of said first level is completed, said series ofbehaviors including: applying a predetermined write voltage between saidcontrol gate and said drain to introduce a charge into said floatinggate and to change the threshold level of said nonvolatile memory cell;applying a voltage responsive to said first level to said control gateand detecting and amplifying a current flowing between the source andthe drain of said nonvolatile memory cell to perform verify reading; andconfirming whether the writing of said first level is finished or not;when a second level of said multi-level data is to be written, repeatinga series of behaviors including a first step and a second step untilwriting of said second value is completed, said first step including:applying said write voltage between said control gate and said drain tointroduce a charge into said floating gate and to change the thresholdvalue of said nonvolatile memory cell by a large amount than that forwriting said first value; applying a voltage responsive to said secondvalue to said control gate and detecting and amplifying a currentflowing between the source and the drain of said nonvolatile memory cellto perform verify reading; and confirming whether the writing of saidfirst value is finished or not, and said second step including: applyingsaid write voltage between said control gate and said drain to introducea charge into said floating gate and to change the threshold value ofsaid nonvolatile memory cell; applying a voltage responsive to saidsecond value to said control gate and detecting and amplifying a currentflowing between the source and the drain of said nonvolatile memory cellto perform verify reading; and confirming whether the writing of saidfirst value is finished or not.
 3. A method for using a nonvolatilesemiconductor memory device including nonvolatile memory cells whicheach have a source and a drain both formed on one surface of asemiconductor substrate, and including a floating gate and a controlgate which are stacked on the semiconductor substrate above a portionbetween the source and the drain via insulation films, comprising: whena first value of multi-level data is to be written, repeating a seriesof behaviors until writing of said first value is completed, said seriesof behaviors including: applying a predetermined write voltage betweensaid control gate and said drain to introduce a charge into saidfloating gate and to change the threshold value of said nonvolatilememory cell; applying a voltage responsive to said first value to saidcontrol gate and detecting and amplifying a current flowing between thesource and the drain of said nonvolatile memory cell to perform verifyreading; and confirming whether the writing of said first value isfinished or not; when a second value of said multi-level data is to bewritten, repeating a series of behaviors until writing of said secondvalue is completed, said series of behaviors including: while writingsaid first value in said nonvolatile memory cell, applying said writevoltage between said control gate and said drain to introduce a chargeinto said floating gate and to change the threshold value of saidnonvolatile memory cell; applying a voltage responsive to said secondvalue to said control gate and detecting and amplifying a currentflowing between the source and the drain of said nonvolatile memory cellto perform verify reading; and confirming whether the writing of saidfirst value is finished or not.
 4. A method for using a nonvolatilesemiconductor memory device which includes a memory cell array having aplurality of nonvolatile memory cells arranged in a matrix in whichmemory cells in one row are connected by a common word line and memorycells in one column are connected by a common bit line, and a pluralityof flag cells each associated with a memory cell group including aplurality of memory cells in said memory cell array to hold the numberof data stored in a single memory cell in the associated memory cellgroup, comprising the steps of: reading a flag data of a flag cell of amemory cell group to which a memory cell to be read belongs; repeating aplurality of cycles of driving a word line connected to the memory cellwith a predetemined voltage pursuant to the flag data of said flag cell,then detecting and amplifying a voltage of a bit line, and outputtingthe read-out data; and converting the data read out in said cycles intobinary data.